The transfer mold encapsulation process has been used with thermoset molding compounds, such as epoxy molding compounds, for about 30 years. Conventional transfer molds used in this process are typically high density molds with as few as 20 cavities to over 800 cavities per molded encapsulation shot. As the demand for higher quality, as imposed by the semiconductor industry continues, encapsulation in high density transfer molds falls short of meeting current quality goals for wire sweep, lead frame paddle tilt and internal voids. All three noted quality defects also potentially can result in reliability problems, especially as semiconductor devices, including integrated circuit devices, become larger in relationship to their molded plastic package size, and as lead counts continue to increase. This is because the cavities containing the devices are filled at different times, with different flow rates and material temperatures, having varying viscosities, because of the single transfer plunger, chase runner, cavity gate geometry and position. This leads to a typical "Christmas tree" pattern during the mold filling process.
Industry, over the years, has taken two primary mold design approaches to improve the quality of encapsulated devices. One approach has been to balance fill the mold by varying the runner cross section (taper) in the mold chase which typically holds two lead frame strips with multiple wire bonded devices. In addition, the gate entry angles into the cavities where the devices are located were also varied with the gate depth being constant. The second approach is to use multiple transfer plungers to fill the cavities containing the wire bonded devices on the strip simultaneously. Both approaches improve the quality of the molded device package, with the multiplunger approach being somewhat superior because of its better fill balance.
In the first approach, the "Christmas tree" effect is minimized, but not eliminated. The material melt temperature is also not consistently the same and therefore the melt viscosity still varies. Since the melt viscosity applies the material force as the material fills the cavity, the noted defects are not totally eliminated.
The second approach, the multiplunger mold, works to a greater degree of success versus the balanced fill approach. It, however, is deficient primarily since the process does not use dielectrically preheated material. As a result, there is a temperature gradient for the material as it fills the cavity. Because of the temperature gradient, the melt viscosity varies during the filling process of each cavity. Filling therefore is not perfectly uniform.